/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.net.bsd;

import java.io.IOException;
import java.io.InputStream;
import java.net.BindException;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;
import java.net.UnknownHostException;
import java.nio.charset.StandardCharsets;

import org.apache.commons.io.IOUtils;
import org.apache.commons.net.io.SocketInputStream;

/**
 * RCommandClient is very similar to {@link org.apache.commons.net.bsd.RExecClient}, from which it is derived, and implements the rcmd() facility that first
 * appeared in 4.2BSD Unix. rcmd() is the facility used by the rsh (rshell) and other commands to execute a command on another machine from a trusted host
 * without issuing a password. The trust relationship between two machines is established by the contents of a machine's /etc/hosts.equiv file and a user's
 * .rhosts file. These files specify from which hosts and accounts on those hosts rcmd() requests will be accepted. The only additional measure for establishing
 * trust is that all client connections must originate from a port between 512 and 1023. Consequently, there is an upper limit to the number of rcmd connections
 * that can be running simultaneously. The required ports are reserved ports on Unix systems, and can only be bound by a process running with root permissions
 * (to accomplish this rsh, rlogin, and related commands usualy have the suid bit set). Therefore, on a Unix system, you will only be able to successfully use
 * the RCommandClient class if the process runs as root. However, there is no such restriction on Windows95 and some other systems. The security risks are
 * obvious. However, when carefully used, rcmd() can be very useful when used behind a firewall.
 * <p>
 * As with virtually all the client classes in org.apache.commons.net, this class derives from SocketClient. But it overrides most of its connection methods
 * so that the local Socket will originate from an acceptable rshell port. The way to use RCommandClient is to first connect to the server, call the
 * {@link #rcommand rcommand()} method, and then fetch the connection's input, output, and optionally error streams. Interaction with the remote command is
 * controlled entirely through the I/O streams. Once you have finished processing the streams, you should invoke
 * {@link org.apache.commons.net.bsd.RExecClient#disconnect disconnect()} to clean up properly.
 * </p>
 * <p>
 * By default, the standard output and standard error streams of the remote process are transmitted over the same connection, readable from the input stream
 * returned by {@link org.apache.commons.net.bsd.RExecClient#getInputStream getInputStream()} . However, it is possible to tell the rshd daemon to return the
 * standard error stream over a separate connection, readable from the input stream returned by {@link org.apache.commons.net.bsd.RExecClient#getErrorStream
 * getErrorStream()} . You can specify that a separate connection should be created for standard error by setting the boolean
 * {@code separateErrorStream} parameter of {@link #rcommand rcommand()} to {@code true}. The standard input of the remote process can be written
 * to through the output stream returned by {@link org.apache.commons.net.bsd.RExecClient#getOutputStream getOutputStream()}.
 * </p>
 *
 * @see org.apache.commons.net.SocketClient
 * @see RExecClient
 * @see RLoginClient
 */
public class RCommandClient extends RExecClient {

    /**
     * The default rshell port. Set to 514 in BSD Unix.
     */
    public static final int DEFAULT_PORT = 514;

    /**
     * The smallest port number a rcmd client may use. By BSD convention this number is 512.
     */
    public static final int MIN_CLIENT_PORT = 512;

    /**
     * The largest port number a rcmd client may use. By BSD convention this number is 1023.
     */
    public static final int MAX_CLIENT_PORT = 1023;

    /**
     * The default RCommandClient constructor. Initializes the default port to {@code DEFAULT_PORT}.
     */
    public RCommandClient() {
        setDefaultPort(DEFAULT_PORT);
    }

    /**
     * Opens a Socket connected to a remote host at the specified port and originating from the current host at a port in a range acceptable to the BSD rshell
     * daemon. Before returning, {@link org.apache.commons.net.SocketClient#_connectAction_ _connectAction_()} is called to perform connection initialization
     * actions.
     *
     * @param host The remote host.
     * @param port The port to connect to on the remote host.
     * @throws SocketException If the socket timeout could not be set.
     * @throws BindException   If all acceptable rshell ports are in use.
     * @throws IOException     If the socket could not be opened. In most cases you will only want to catch IOException since SocketException is derived from
     *                         it.
     */
    @Override
    public void connect(final InetAddress host, final int port) throws SocketException, IOException {
        connect(host, port, InetAddress.getLocalHost());
    }

    /**
     * Opens a Socket connected to a remote host at the specified port and originating from the specified local address using a port in a range acceptable to
     * the BSD rshell daemon. Before returning, {@link org.apache.commons.net.SocketClient#_connectAction_ _connectAction_()} is called to perform connection
     * initialization actions.
     *
     * @param host      The remote host.
     * @param port      The port to connect to on the remote host.
     * @param localAddr The local address to use.
     * @throws SocketException If the socket timeout could not be set.
     * @throws BindException   If all acceptable rshell ports are in use.
     * @throws IOException     If the socket could not be opened. In most cases you will only want to catch IOException since SocketException is derived from
     *                         it.
     */
    public void connect(final InetAddress host, final int port, final InetAddress localAddr) throws SocketException, BindException, IOException {
        int localPort;

        for (localPort = MAX_CLIENT_PORT; localPort >= MIN_CLIENT_PORT; --localPort) {
            try {
                _socket_ = _socketFactory_.createSocket(host, port, localAddr, localPort);
                break;
            } catch (final SocketException e) {
                continue;
            }
        }

        if (localPort < MIN_CLIENT_PORT) {
            throw new BindException("All ports in use or insufficient permssion.");
        }

        _connectAction_();
    }

    /**
     * Opens a Socket connected to a remote host at the specified port and originating from the specified local address and port. The local port must lie
     * between {@code MIN_CLIENT_PORT} and {@code MAX_CLIENT_PORT} or an IllegalArgumentException will be thrown. Before returning,
     * {@link org.apache.commons.net.SocketClient#_connectAction_ _connectAction_()} is called to perform connection initialization actions.
     *
     * @param host      The remote host.
     * @param port      The port to connect to on the remote host.
     * @param localAddr The local address to use.
     * @param localPort The local port to use.
     * @throws SocketException          If the socket timeout could not be set.
     * @throws IOException              If the socket could not be opened. In most cases you will only want to catch IOException since SocketException is
     *                                  derived from it.
     * @throws IllegalArgumentException If an invalid local port number is specified.
     */
    @Override
    public void connect(final InetAddress host, final int port, final InetAddress localAddr, final int localPort)
            throws SocketException, IOException, IllegalArgumentException {
        if (localPort < MIN_CLIENT_PORT || localPort > MAX_CLIENT_PORT) {
            throw new IllegalArgumentException("Invalid port number " + localPort);
        }
        super.connect(host, port, localAddr, localPort);
    }

    /**
     * Opens a Socket connected to a remote host at the specified port and originating from the current host at a port in a range acceptable to the BSD rshell
     * daemon. Before returning, {@link org.apache.commons.net.SocketClient#_connectAction_ _connectAction_()} is called to perform connection initialization
     * actions.
     *
     * @param hostname The name of the remote host.
     * @param port     The port to connect to on the remote host.
     * @throws SocketException      If the socket timeout could not be set.
     * @throws BindException        If all acceptable rshell ports are in use.
     * @throws IOException          If the socket could not be opened. In most cases you will only want to catch IOException since SocketException is derived
     *                              from it.
     * @throws UnknownHostException If the hostname cannot be resolved.
     */
    @Override
    public void connect(final String hostname, final int port) throws SocketException, IOException, UnknownHostException {
        connect(InetAddress.getByName(hostname), port, InetAddress.getLocalHost());
    }

    /**
     * Opens a Socket connected to a remote host at the specified port and originating from the specified local address using a port in a range acceptable to
     * the BSD rshell daemon. Before returning, {@link org.apache.commons.net.SocketClient#_connectAction_ _connectAction_()} is called to perform connection
     * initialization actions.
     *
     * @param hostname  The remote host.
     * @param port      The port to connect to on the remote host.
     * @param localAddr The local address to use.
     * @throws SocketException If the socket timeout could not be set.
     * @throws BindException   If all acceptable rshell ports are in use.
     * @throws IOException     If the socket could not be opened. In most cases you will only want to catch IOException since SocketException is derived from
     *                         it.
     */
    public void connect(final String hostname, final int port, final InetAddress localAddr) throws SocketException, IOException {
        connect(InetAddress.getByName(hostname), port, localAddr);
    }

    /**
     * Opens a Socket connected to a remote host at the specified port and originating from the specified local address and port. The local port must lie
     * between {@code MIN_CLIENT_PORT} and {@code MAX_CLIENT_PORT} or an IllegalArgumentException will be thrown. Before returning,
     * {@link org.apache.commons.net.SocketClient#_connectAction_ _connectAction_()} is called to perform connection initialization actions.
     *
     * @param hostname  The name of the remote host.
     * @param port      The port to connect to on the remote host.
     * @param localAddr The local address to use.
     * @param localPort The local port to use.
     * @throws SocketException          If the socket timeout could not be set.
     * @throws IOException              If the socket could not be opened. In most cases you will only want to catch IOException since SocketException is
     *                                  derived from it.
     * @throws UnknownHostException     If the hostname cannot be resolved.
     * @throws IllegalArgumentException If an invalid local port number is specified.
     */
    @Override
    public void connect(final String hostname, final int port, final InetAddress localAddr, final int localPort)
            throws SocketException, IOException, IllegalArgumentException, UnknownHostException {
        if (localPort < MIN_CLIENT_PORT || localPort > MAX_CLIENT_PORT) {
            throw new IllegalArgumentException("Invalid port number " + localPort);
        }
        super.connect(hostname, port, localAddr, localPort);
    }

    /**
     * Overrides method in RExecClient in order to implement proper port number limitations.
     */
    @Override
    InputStream createErrorStream() throws IOException {
        final Socket socket;
        try (ServerSocket server = createServer()) {
            _output_.write(Integer.toString(server.getLocalPort()).getBytes(StandardCharsets.UTF_8));
            _output_.write(NULL_CHAR);
            _output_.flush();
            socket = server.accept();
        }
        if (isRemoteVerificationEnabled() && !verifyRemote(socket)) {
            final String hostAddress = getHostAddress(socket);
            IOUtils.closeQuietly(socket);
            throw new IOException("Security violation: unexpected connection attempt by " + hostAddress);
        }
        return new SocketInputStream(socket, socket.getInputStream());
    }

    private ServerSocket createServer() throws IOException {
        for (int localPort = MAX_CLIENT_PORT; localPort >= MIN_CLIENT_PORT; --localPort) {
            try {
                return _serverSocketFactory_.createServerSocket(localPort, 1, getLocalAddress());
            } catch (final SocketException e) {
                continue;
            }
        }
        throw new BindException("All ports in use.");
    }

    /**
     * Same as {@code rcommand(localUserName, remoteUserName, command, false);}
     *
     * @param localUser  the local user
     * @param remoteUser the remote user
     * @param command        the command
     * @throws IOException on error
     */
    public void rcommand(final String localUser, final String remoteUser, final String command) throws IOException {
        rcommand(localUser, remoteUser, command, false);
    }

    /**
     * Remotely executes a command through the rshd daemon on the server to which the RCommandClient is connected. After calling this method, you may interact
     * with the remote process through its standard input, output, and error streams. You will typically be able to detect the termination of the remote process
     * after reaching end of file on its standard output (accessible through {@link #getInputStream getInputStream()}). Disconnecting from the server or closing
     * the process streams before reaching end of file will not necessarily terminate the remote process.
     * <p>
     * If a separate error stream is requested, the remote server will connect to a local socket opened by RCommandClient, providing an independent stream
     * through which standard error will be transmitted. The local socket must originate from a secure port (512 - 1023), and rcommand() ensures that this will
     * be so. RCommandClient will also do a simple security check when it accepts a connection for this error stream. If the connection does not originate from
     * the remote server, an IOException will be thrown. This serves as a simple protection against possible hijacking of the error stream by an attacker
     * monitoring the rexec() negotiation. You may disable this behavior with {@link org.apache.commons.net.bsd.RExecClient#setRemoteVerificationEnabled
     * setRemoteVerificationEnabled()}.
     * </p>
     *
     * @param localUser       The user account on the local machine that is requesting the command execution.
     * @param remoteUser      The account name on the server through which to execute the command.
     * @param command             The command, including any arguments, to execute.
     * @param separateErrorStream True if you would like the standard error to be transmitted through a different stream than standard output. False if not.
     * @throws IOException If the rcommand() attempt fails. The exception will contain a message indicating the nature of the failure.
     */
    public void rcommand(final String localUser, final String remoteUser, final String command, final boolean separateErrorStream) throws IOException {
        rexec(localUser, remoteUser, command, separateErrorStream);
    }

}
